Why are some items not showing up in my rendered animation?
When I open your file and play back the animation (by pressing [Alt+A]) the Ts do not move at all. They hang above the camera and never come into the picture, thus they never show up in your render.
I am not sure why the animation behaves like this, but I have found a way to resolve the problem: Set the Start frame of your animation to 1 and let it play once. The rigid bodies will fall down from their starting position. When you set it back to 250 afterwards, the Ts will behave correctly and fall down.

Edit: It looks like Blender is caching the animation. See the orange line at the bottom of the Timeline:

The part that is cached is colored in a strong orange, the uncached part is only slightly orange. I think this cache is temporary and is lost when you close blender. If you start your animation again from a point that is not the beginning, Blender does not know the current velocities / angular velocities of the rigid bodies and could not solve the rigid body motion correctly, so it shows no motion at all instead.